A membrane filter having capillary membranes of the membrane filter that can be used in immersion operation that are clamped between two head pieces at their ends is known from WO 98/28066. A gasification device is connected at the one head piece, which is the lower one in operation. The other, upper end contains the permeate collecting chamber. It forms a displacement body, which has the effect of displacing the rising air bubbles towards the outside of the membrane fiber bundle. Effective gasification is no longer guaranteed in the upper region of the fiber bundle. Here, no membrane movements are possible either, or only slight membrane movements are possible. As a consequence, the formation of membrane cover layers and of gross dirt deposits occurs in the upper region of the fiber bundle.
In a membrane filter known from DE 198 11 945 A1, the fiber bundle of capillary membranes is set into a pressure-resistant mantle pipe, through which the flow passes in the lengthwise direction of the capillary membranes. The capillary membranes are secured into an in-flow base at their ends, which base has a plurality of bores uniformly distributed over its cross-section, to allow the untreated water to flow through. In order to prevent cover layers from forming on the outside of the capillary membranes, and to prevent the micropores of the capillary membranes from becoming blocked, a uniform flow through the membrane filter chamber, at a defined flow velocity, is required. In order to improve the material exchange, the untreated water can be gasified with air before it enters into the membrane filter chamber. Sufficient air distribution within the fiber bundle is not guaranteed. The air is essentially guided along the outside of the membrane bundle, with the untreated water flow, and does not make any effective contribution to the membrane purification effect.